Keeping the Standard Kilogram From Gaining Weight Is a Constant Struggle

Peter Cumpson has come up with a simpler, more reproducible way to clean standard kilograms. Here, he's pictured with the device used to measure contamination on the surface of metals. (Mike Urwin)

After more than a century, the international prototype kilogram – a cylindrical chunk of metal stored in a French vault – doesn’t weigh the same as its 40 replicas, distributed worldwide and used to standardize mass measurements. Suspecting that gunk accumulating on the metallic surfaces is to blame, scientists at Newcastle University have developed a high-tech way to clean the standards.

If the washing protocol makes it into labs around the world, it could help with the problem of drifting masses – at least until a different standard kilogram is developed, one that relies on a fundamental physical constant rather than on an actual thing.

“It sounds good,” said Richard Davis, a physicist and former head of the Mass Section at the International Bureau of Weights and Measures in France. “The technique they’re proposing is something that is not that expensive and could be implemented in different places without too much trouble.”

Forged from platinum and iridium in London, the official international standard has, since 1889, been stored in a vault near Paris belonging to the International Bureau of Weights and Measures. In 1884, 40 replicas of the roughly 2.2-pound cylinder were made; in 1889, 34 of the replicas were sent to countries around the world.

They were supposed to be the kilo’s twins.

But in the late 1980s, scientists noticed that the original kilogram was about 50 micrograms lighter than its brethren. Because mass measurements are relative, it’s tough to determine whether the replicas are getting heavier or the original is getting lighter. Peter Cumpson, a metrologist at Newcastle, suspects that a good cleaning could help restore balance to the masses. But the cleaning of a standard must, of course, be standard and reproducible. Though the metal cylinders are bathed fairly regularly, the process involves hand-washing, which introduces mechanical rubbing that’s hard to reproduce; what scientists need is “a much more repeatable, controllable, reproducible method of cleaning these kilograms,” Cumpson said.

Over the last two decades, Cumpson and others have developed such a method. It’s based not on manually scrubbing the metal chunks but on exposing them to ultraviolet light and ozone about once per decade. Recently, Cumpson fine-tuned the procedure and added another step: a pure water rinse to remove dust particles. The final recipe is now available online in the journal Metrologia.

“I think we’ve cracked it,” Cumpson said.

Cumpson tested the process using prototype metals, including gold, stored under conditions similar to the British national standard. The ultraviolet light-ozone treatment removes hydrocarbon contamination that has built up on the metal surface, gunk that comes from the emissions of an industrial society. Cumpson suspects that because the kilos living in national labs have been retrieved and handled more frequently than the international kilo, more carbon-containing contaminants have built up on them over time. Incubating the kilograms with a set amount of ozone and ultraviolet light “gently breaks up the carbonaceous contamination at the surface,” Cumpson says. “The majority product is CO2, which will simply diffuse away.”

The new kilogram? Scientists working on redefining the kilogram have crafted silicon spheres containing a specific numbers of atoms. (CSIRO/Wikimedia Commons)

There’s another kilo-contaminant, though, one that sticks too stubbornly to remove: Mercury atoms from trace amounts of free mercury in labs that once used mercury-based thermometers. “Mercury’s a most unfortunate contaminant,” Cumpson said. “It’s difficult to remove, it forms a strong metallic bond with platinum.”

Even now, the UV-ozone method has only been tested on prototype metals, but it’s close to being used on the actual standards, Cumpson said. The International Bureau of Weights and Measures, and well as Britain’s National Physics Laboratory, have already installed some of the necessary machinery. But that’s not all the French lab is doing: In the meantime, the facility is testing newer, better ways to store the resident mass standards, including such things as special storage containers, vacuums, and pure, flowing argon or nitrogen atmospheres.

“These are pretty good standards for the reasonably long-term, but not for eternity,” Davis said. “But over the short run, they’re quite good. If we can clean them reproducibly.”

Eventually, the kilogram will be redefined, based on a fundamental natural constant and not something variable like a chunk of metal. So far, the constants in the running are Avogadro’s – related to the mass of an atom – and Planck’s – used in quantum physics and related to the energy of a photon and resulting electromagnetic wave. To realize the new kilogram, scientists have been crafting pure silicon spheres containing a fixed number of atoms, as well as performing experiments using a watt balance, which balances electric forces against the gravity pull on a kilogram. “In the next several years, we’ll have it sorted out,” Davis said. “Whichever one works, you can use. But they’ve got to give you the same result.”